Aerogels are unique high surface area materials that have promise as insulators, low dielectric substrates, low density core materials for sandwich structures and chemical separation media in applications where low density and low weight are desirable. Aerogels were first produced by using supercritical fluid extraction to remove liquid from a variety of gels, including silica, gelatin, agar, cellulose, and nitrocellulose, and replacing it with air. The first synthetic polymer aerogels were based on polycondensation of formaldehyde with resorcinol or melamine. Similar to inorganic alumina and silica aerogels, they possess high surface areas, low densities, and low thermal conductivity but are brittle and have poor mechanical properties.
Polymer aerogels of many types have been synthesized by removing the liquid from a polymer gel by some means. Gels formed by crystallization of linear polymers in solution are typically prepared by cooling hot solutions of polymers such as polyvinylidene fluoride (PVDF), poly(4-methyl-pentene-1) (i-P4MP1), and syndiotactic polystyrene (s-PS). Factors such as solvent choice and cooling rate dictate the types of crystalline morphologies and the amount of fibrous, amorphous regions that are present. Because of the controlled combination of crystalline and amorphous regions and the high porosity, syndiotactic polystyrene (s-PS) aerogels are valued as absorbents for volatile organic compounds.
Interest in improving the mechanical stability of aerogels has led to the development of covalently cross-linked aerogels composed of polymers such as polyurea, polyurethane, polyimide, and polyamide. The process for fabrication of covalently cross-linked gels typically begins with forming telechelic oligomers that gel after addition of a suitable cross-linker. This typically gives polymers with tailorable properties depending on the oligomer backbone and the cross-linker. For example, polyimide aerogels have been fabricated as thin films with good moisture resistance, as mechanically strong materials, and with low dielectric constants and demonstrated as substrates for lightweight antennas.
To fabricate cross-linked polyimide or polyamide aerogels, reactions are carried out in polar aprotic solvents at room temperature or lower and entail the condensation of bisnucleophiles with biselectrophiles to form step-growth oligomers. Control of the stoichiometric balance between the nucleophiles and electrophiles allows for control over the number of repeat units, n, of the oligomers formed in solution. Furthermore, it is possible to form oligomers end-capped with either two electrophilic or two nucleophilic sites based on the molar excess of bisnucleophile or biselectrophile. Cross-linkers for electrophilic end groups such as anhydrides or isocyanates have included aromatic triamines, such as 1,3,5-triaminophenoxybenzene (TAB), 1,3,5-tris(aminophenyl)benzene, 2,4,6-tri(aminophenyl)pyridine, and octa-aminophenylsilses-quioxane (OAPS). If the oligomers are capped with nucleophiles, such as amines, a reagent with three or more electrophilic moieties, such as 1,3,5-benzenetricarbonyl trichloride (BTC) or poly(maleic anhydride) can be used as a cross-linker to react with the end groups of the oligomers to create a three dimensional network which forms the gel.
Polyamide aerogels, produced as described above, using the inexpensive monomers, isophthaloyl chloride (IPC) and p-phenylene diamine (PPDA) and cross-linked with BTC had the highest Young's moduli reported to date for a polymer aerogel, compared on a same density basis. Furthermore, these linear crosslinked materials could be made without the use of an inert atmosphere, unlike previously reported polyamide aerogels made using isocyanates.